A DIY Alkalinity Test

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Randy Holmes-Farley

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A DIY Alkalinity Test

diy-alk.jpg


Alkalinity is one of the most important measurements that a reef aquarist can make. It can become rapidly depleted in many aquaria, requiring frequent measurement in order to maintain stable levels.

While hobby test kits for alkalinity can be simple to use, some aquarists find them either tedious to perform, expensive, or of questionable accuracy. This article details how one can perform DIY alkalinity tests that do not involve visualizing color changes,...
Read more about this article here...
 
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L Driehuis

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Thanks for this article!

Is it possible to return the water to the aquarium when done with the test?
Is it also possible to use a weak acid like acetic acid?
 
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Randy Holmes-Farley

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I would not add it back since it becomes zero alkalinity seawater, but it makes no big difference to the tank since you do not use much.

I don't think a weak acid like acetic will work because its pKa is higher than the endpoint (at least in fresh water, it may be lower in seawater). So when you get to pH 4.76 (the freshwater pKa), half of the acetic acid you are adding will stay in the acid form and not release its proton to react with bicarbonate. One likely could still do it this way with a lot of calculating and knowing the exact pKa in seawater, but it would be tricky.
 

L Driehuis

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Thanks for your reply. I suspected already that a weak acid is going to be tricky. The Ph drop will be subtle.

Interesting, it looks like this process can be easily automated using dosing pumps. The process will be better in case the resulting water can be returned.
Do you think the water can be returned if enough sodium bicarbonate is added to compensate?
 
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Randy Holmes-Farley

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Thanks for your reply. I suspected already that a weak acid is going to be tricky. The Ph drop will be subtle.

Interesting, it looks like this process can be easily automated using dosing pumps. The process will be better in case the resulting water can be returned.
Do you think the water can be returned if enough sodium bicarbonate is added to compensate?
Yes. There's a thread discussing this sort of automation and the return of the tank water or not:

https://www.reef2reef.com/threads/kh-guardian-khg-alk-calculation.277092
 
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chipmunkofdoom2

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Hey Randy, I had a question about the method above. I did some rough calculations based on some numbers I found on other forums, and it appears you're correct, the acid you add to the samples should effectively react with all the bicarbonate and reduce the levels to 0 dKh. What if you dosed the sample water to return it to its original alkalinity?

Let's say you test a 500mL sample and determine its alkalinity to be around 8 dKh. Using your DIY sodium bicarbonate solution (which you stated was around 2,660 dKh), I've calculated it would take around 1.51 mL of the solution to return the sample to a value of around 8 dKh. Would there be any long term consequences of doing this? I'm just thinking about cases where I might test alkalinity multiple times a day for several days in a row.
 

JimWelsh

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You would, over time, be slightly increasing the sodium and chloride (if you used HCl for the strong acid) or sulfate (if you used H2SO4 for the strong acid) in your tank, but you should put heavy emphasis on the word "slightly". Certainly much less than the NaCl residual left over from dosing Randy's two-part. Short term, though, assuming the volume of acid shown in Randy's article's Table 1 is what is used, you would also be returning a larger volume of diluted water. If your 500 mL of tank water was 35 PPT to begin with, the 515.84 mL of water you would be returning would be right about 34 PPT.
 
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chipmunkofdoom2

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Thanks @JimWelsh for some great info.

Just to clarify, in your last sentence when discussing dilution, you mention 35ppm and 34ppm. Did you mean 35ppt and 34ppt, as in salinity? That would make sense.
 

bubbasguppies

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I know this is an old article. But was wondering one thing. If I go ahead and do the 250mL test and break it down into 1/5th of the original volume that will leave me with 50mL test. Now on doing so with the water test volume I know i've got to dilute the HCL by 1/5th. So, is that 4-parts RO to 1-part HCL or is it 5-parts RO to 1-part HCL? Thanks guys.....awesome read btw @Randy Holmes-Farley .
 

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If I understand your question correctly, then to dilute the titrant to 1/5 of the original concentration, you want 1 part original titrant in 5 parts of total diluted titrant, so it would be 1 part HCl plus 4 parts RODI water for a total of 5 parts of diluted titrant.
 
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Randy Holmes-Farley

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I read this article recently. Glad to see it being discussed again.

quick question #Randy Holmes-Farley

Is reagent degradation a significant concern here, or does the acid age pretty well?
What steps might one take to keep it fresh and accurate?
If it is sealed against evaporation, it does not degrade.
 

Dkeller_nc

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I know this is an old article. But was wondering one thing. If I go ahead and do the 250mL test and break it down into 1/5th of the original volume that will leave me with 50mL test. Now on doing so with the water test volume I know i've got to dilute the HCL by 1/5th. So, is that 4-parts RO to 1-part HCL or is it 5-parts RO to 1-part HCL? Thanks guys.....awesome read btw @Randy Holmes-Farley .
In the article, Randy notes that for 1 liter of aquarium water at a seawater alkalinity (presumably 7 dKH, which he's mentioned elsewhere as a reasonable standard value) the titration will require 25 mL of 0.1N HCl.

So if you're using 50mL of aquarium water, and the alkalinity of the sample is 7 dKH, then you'd need approximately 1.25 mL of 0.1N HCl to titrate that sample to pH 4.5. This volume will be rather easily measured using a disposable plastic syringe such as the ones that are included in the Salifert tests. You can certainly dilute the 0.1N HCl by 80% to get 0.02N HCl so that you'll be adding 6.25mL for that same sample, but if you choose to do so, you will want to do this accurately. This is a subjective judgement, but if you want to do this on a regular basis, I would consider purchasing some laboratory grade glassware for the purpose.

That's quite inexpensive compared to days gone by when I was in undergrad/grad school. All you need is a 1 L class-A volumetric flask and a 250mL graduated cylinder. Total investment - $27 (plus shipping if you don't have Amazon Prime). Several other pieces of laboratory equipment makes doing titrations considerably more convenient. Specifically, a 25mL graduated buret or 10mL buret, a ringstand and buret clamp. A further convenience enhancement is a magnetic stir plate and stir bar. The magnetic stir plate and a 1 or 2 liter volumetric flask makes mixing 2-part solutions really convenient, as well, though none of the above is truly necessary.
 
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Randy Holmes-Farley

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I just saw this. Any reason to use a pH meter over Methyl red or another indicator?
You certainly can use an appropriate indicator (like an alk titration kit does), but a pH meter will allow much better location of the endpoint. There are tons of threads asking what exact color each alk kit should be st the endpoint.

You need to be in the low to mid 4’s. Some alk kits use multiple indicator dyes to more precisely locate it by eye.
 
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